1. Field of the Disclosure
The present invention relates to the manufacture of an aerofoil having a filled cavity, in particular the manufacture of an aerofoil component having a filled cavity for use in a gas turbine engine.
2. Description of the Related Art
Aerofoil shaped components are used throughout gas turbine engines. For example, aerofoil shaped stator vanes and rotor blades are used to guide gas through the engine, for example both in the turbine and the compressor, including the fan and associated guide vanes.
Weight reduction is an important consideration in gas turbine engines, particularly, although not exclusively, for gas turbine engines used to power aircraft. Generally, the lower the weight of the component the better the performance of the aircraft to which it is fitted, for example in terms of fuel consumption. To this end, it is known to use hollow aerofoils, e.g. rotor blades and/or stator vanes, in some stages of gas turbine engines. The hollow aerofoils have an internal cavity, which may be filled with a lightweight material.
One method of producing a hollow aerofoil involves forming the structure using a skin. This involves creating an internal cavity using hot creep or super plastic forming processes. Such processes may generate aerofoils with some advantageous properties, such as thin skin thickness and tight dimensional tolerance, but they involve significant material wastage. This material wastage makes these processes expensive, due at least to high material cost for a given size of hollow aerofoil component.
An alternative method for producing hollow aerofoil components involves attaching a plate to an aerofoil structure out of which a pocket has been machined. The plate is placed into the pocket and attached (for example welded or bonded) therein to produce a hollow aerofoil component.
An advantage of producing the hollow aerofoil by using a plate to cover a pocket in an aerofoil structure is that there is less material wastage than using a skin to produce the hollow aerofoil. However, the dimensional, tolerances are not so accurate. This may be because distortion is introduced in the process of attaching the plate to the pocketed aerofoil, which typically involves local heating at the interface between the plate and the pocketed aerofoil. Additionally, tolerance errors may “stack-up” in the process used to produce the pocketed aerofoil, the process used to produce the plate, and the process/feature used to locate the plate into position in the pocket, which typically involve placing the plate onto a supporting ledge inside the pocket.
The lack of dimensional accuracy means that the plate generally has to be manufactured to be thicker than would otherwise be required. For example, the extra thickness may be required in order to ensure that there is enough material to be machined back to produce the desired aerofoil shape after it has been fixed into the pocket. Without the extra thickness, the dimensional variation resulting from tolerance “stack-up” and/or distortion may mean that there is not sufficient material to produce the desired aerofoil shape in some of the aerofoils produced by the method.
However, this extra thickness means both that the component is heavier than it would otherwise need to be, and also that there is more material wastage.
Regardless of how the aerofoil is formed, the internal cavity is often filled, both to support the aerofoil structure and to prevent moisture ingress to the cavity. The filler is conventionally a lightweight, non-expandable material, such as syntactic epoxy.
Conventionally, such fillers are injected into the cavity after it is formed, for example by forming the hollow aerofoil using one of the methods outlined above. This means that only fillers which are injectable can be used to fill the cavity. Furthermore, to ensure the panels do not bulge due to internal pressure, foaming/expandable materials cannot be used. Even with judicious positioning of the filling/injection holes, it is not possible to completely fill the internal cavity using such injection.
Furthermore, injecting a filler material into a cavity necessarily results in a filler with homogeneous properties, whereas some composite or foamed structures, which are not injectable, may be stiffer, have lower density and better damping than such homogeneous fillers. As such, the type and selection of filler materials which can be used is limited, and materials which may have advantageous properties may be precluded from use in such an injection filling process
Furthermore, filling the cavity by injecting the filler into injection holes after the hollow aerofoil has been formed means that the cavity must be defined such that the filler material can be pumped in from one end and vent out the other end so as to ensure that the cavity is filled as completely as possible. As such any internal pocket definition of the aerofoil, such as stiffening ribs, is compromised because of the need to ensure that such features do not obstruct the flow of the filler, which may result in an unacceptable level of fill.